It is important for the continued successful utilization of diamond tools that such tools be capable of withstanding extremely severe conditions of use without the diamond cutting surface breaking free from the tool. Diamond cutting tools are commonly prepared by adhering a diamond cutting element to an extremely hard and durable tool substrate material such as tungsten carbide. For flat tools, such as those intended for use on a metal lathe, the diamond cutting elements are typically in the form of small triangular "inserts" which are clamped in place on a larger, bar-shaped "tool holder" which is held in a lathe or similar machine during actual use. Traditionally such inserts have been based upon a polycrystalline compact diamond (PCD) body which has tungsten carbide on one surface. Polycrystalline diamond compacts are a mixture of a binder such as cobalt and diamond powder. Such diamond compact materials are relatively thick, e.g. about 0.080" (2.03 mm), and relatively short in length, e.g. about 0.065" (1.65 mm) maximum. To form a cutting element, the tungsten carbide surface of the PCD is brazed to a larger tungsten carbide body.
In many applications, however, polycrystalline compact diamonds have been found unacceptable due to the presence of the cobalt or for insufficient hardness. Chemical vapor deposited (CVD) diamond film does not contain cobalt which can increase surface friction as well as be a contaminant for a workpiece. Moreover, CVD diamond film is harder than a PCD body. Accordingly, there is an increasing desire to utilize diamond film to form cutting elements. To do so in a manner which allows the direct commercial substitution for CVD diamond for PCD, however, requires adhering CVD diamond to a tungsten carbide strip so that it can be subsequently joined to a larger tungsten carbide body by a braze.
Due to the high temperatures experienced by the cutting element both in use as well as during adherence of one tungsten carbide surface to another, high temperature brazes have been used to join a diamond film to tungsten carbide. However, CVD diamond films do not readily adhere to tungsten carbide, nor many other surfaces, and thus the art has previously used brazing alloys which contain metals which will improve the wetting of the braze alloy on the diamond film, generally by reacting with the carbon thereof to form carbides. Such metals have been referred to in the art as "active metals" or "reactive metals" and include such as titanium, tantalum, chromium, nickel, and the like. Prior to this invention, the art believed that the best active metal for adhering a CVD diamond film was titanium due-to its known ability to react with carbon and form titanium carbide (TIC). Such alloys are generally referred to in the brazing art as "active metal brazes" or "active brazing alloys" or "reactive brazing alloys" or "reactive metal brazing alloys."
Specifically, copending application Ser. No. 07/848,617 filed Oct. 8, 1993, entitled "Diamond Film Cutting Tool" (Attorney Docket DF-2624), assigned to the assignee of this application, discloses the brazing of a diamond film to a tungsten carbide surface by standard active brazing techniques with a reactive metal braze alloy based upon silver, gold, palladium, and the like. In addition to the above metals, the braze further contained a metal capable of forming a carbide thereof at the interface with the diamond film, such as Ti, Ta, Cr, and Mn. The braze used in the Examples was an ordinary silver-copper braze containing Ti or Ta as the active/reactive metal.
U.S. Pat. No. 5,020,394 discloses brazing a CVD diamond film to the rake face of a tool base by means of a two-step brazing process which is both expensive and results in a poor quality bond.
Saint-Gobain Corporation currently manufactures diamond film cutting elements wherein a CVD diamond film is joined to tungsten carbide by means of a commercially available reactive metal braze alloy (Lucanex 721 from Lucas-Milhaupt Inc., 5656 South Pennsylvania Avenue, Caudahy, Wis. 53110), which contains 72 parts silver, 28 parts copper, and 5 to 10 parts titanium.
It is an object of the present invention to improve the reliability of the bond between a diamond film cutting element and a tungsten carbide substrate to which the cutting element is attached.